1. Field of the Invention
The present invention relates to a method of molding a composite insulator utilizing a metal mold and a metal molding apparatus used for this molding method.
In the present invention, "composite insulator" has a core member made of fiber reinforced plastics and an overcoat portion made of insulation polymer materials arranged on an outer surface of the core member, and it means a wide concept including not only polymer insulator in which the core member is formed by an FRP solid member but also polymer hollow insulator or hollow SP insulator in which the core member is formed by an FRP cylindrical member. Moreover, the overcoat portion generally has a cylindrical sheath portion arranged on an outer surface of the core member and a plurality of sheds arranged on an outer surface of the sheath portion with a predetermined interval. Further, as insulation polymer materials, silicone rubber, ethylene-propylene copolymer (RPM), ethylene-propylene-diene copolymer (EPDM) and so on are preferably used.
2. Related Art Statement
One example of a known method of molding a composite insulator in described hereinafter. In the case of molding a polymer insulator by utilizing a metal mold, the polymer insulator is molded by setting with pressure a core member having for example flange metal fittings at both ends thereof, in the metal mold heated preliminarily at a predetemined temperature by: using a heating plate for example arranged outside of the metal mold, via a sheet made of overcoat portion forming materials such as silicone rubber; filing overcoat portion forming materials into a cavity for forming the overcoat portion i.e. sheath portion and sheds, the cavity being generated between the core member and the metal mold; and maintaining with pressure the sheet for a predetermined time interval. In this manner, the overcoat materials are cured and hardened, and an upper metal sold portion and a lower metal mold portion rich construct the metal mold are detached with each other so an to obtain the polymer insulator. In the example mentioned above, a compression molding method was explained. However, also in the another molding method such as an injection molding method and a transfer molding method, the heating method for curing overcoat forming materials is same while the overcoat forming material filling method is different.
FIG. 4 is a schematic view showing one embodiment of a metal molding apparatus used for performing the known method of molding a polymer insulator. In the embodiment shown in FIG. 4, a numeral 51 is a metal mold which is constructed by a pair of an upper metal mold portion and a lower metal mold portion. A core member 53 having flange metal fittings 52 at both ends is set in the metal mold 51. A cavity 54 for molding a sheath portion and sheds is formed between the metal mold 51 and the core member 53. In the cavity 54, overcoat portion forming materials 55 such as silicone rubber are filled. Heating plates 56 are arranged outside of the metal mold 51 and the metal mold 51 can be uniformly heated by the heating plates 56.
In the known molding method mentioned above, the flange metal fittings 52 having a large heat capacity are arranged at both ends of the core member 53, and the flange metal fittings 52 and the core member 53 are heated in the metal mold 51 at the same time during the molding step. Generally, this heating step is performed for a time interval during which the coat portion forming material 55 is sufficiently cured. In this time, the overcoat portion forming material 55 arranged at a portion near a center of the metal mold 51 is sufficiently cured, but the overcoat portion forming material 55 arranged at a portion near the flange metal fittings 52 is not sufficiently cured. If such an insufficient curing portion exists, there is a case that respective members are not connected sufficiently.
The present inventor investigated in detail the insufficient connection mentioned above. By the inventor's investigations, it was found that a temperature increase of a portion near the flange metal fitting 52 is low as cared with that of the other portion. From this point of view, various investigations were performed so as to uniformly cure the overcoat portion forming materials by making a temperature increase of the overcoat portion forming materials arranged at a portion near to the flange metal fitting 52 equal to that of the center portion.
At first, the inventor tried to make a heating time for the curing operation longer than the normal case in response to a low temperature increase of a portion near the flange metal fitting 52. However, in this case, a total curing time was increased. Therefore, it was found that such a total curing time increase was little for one molding operation but there was a large problem on the curing time if a large number of polymer insulators are molded in a large scale manufacturing. Then, the inventor tried to make a temperature increase of a portion near the flange metal fitting 52 faster, while the curing time was maintained normally, by making an overall temperature of the metal mold 51 higher than a normal curing temperature. However, in this case, it was found that a so-called scorch, in which a rough portion was generated on a surface of the product, occurred. Moreover, the inventor tried to make a preliminarily heating temperature of the flange metal fittings 52 and the core member 53 higher than the normal preliminarily heating temperature, when the core member 53 with the flange metal fittings 52 was set in the metal mold 51. However, in this case, it was found that the core member 53 was softened and a handling of the core member 53 before the molding operation became very difficult.